Deep Microbial Colonization in 2-Billion-Year-Old Ultramafic Rock from the Bushveld Complex

Archean cratons may provide stable microbial habitats in the deep subsurface, as evidenced by the discovery of billion-year-old crustal fluids ^1,2 . However, the long-term habitability of these cratonic environments is uncertain, as polymetamorphic evolution in most cratons typically destroys microbial habitats through mineral reactions and porosity loss ^3,4 . Preservation of deep microbial habitats is more likely where mantle-derived magma intruded the craton after metamorphic overprinting ⁴ . Here we report the discovery of dense microbial colonization at 814 m depth within the 2.05-billion-year-old, unmetamorphosed Bushveld Igneous Complex intrusion, South Africa ⁵ . Using advanced contamination-control protocols ^6,7 and synchrotron-based X-ray spectroscopy, we identified indigenous microbial cells localized at the rims of phlogopite, a hydrous phyllosilicate mineral. Our study reveals that microbial colonization is associated with Fe(III) derived from the structure of phlogopite, where the dehydrogenation likely oxidizes Fe(II) to Fe(III) coupled to H ₂ generation ⁸ . Despite the absence of fracture-driven fluid ingress in the unfractured rock matrix, aqueous alteration evidenced at the rims by potassium removal indicates a self-sustaining habitat driven by an internal redox gradient ⁹ . These findings demonstrate that aqueous alteration of ultramafic rocks can sustain isolated microbial life over geological timescales, significantly expanding the potential for long-term habitability on both Earth and Mars ^4,10 .